Field of the Invention
The present invention relates to a method for hydrotreating hydrocarbon-based charges.
The ready availability of large quantities of hydrogen in refining plants and the increasing demand for improved quality products have led to a considerable development in hydrotreating methods.
Depending upon the operating parameters, hydrotreating eliminates impurities such as nitrogen, sulphur and metals, to hydrogenizes olefins, diolefins and even aromatics or even leads to the rupture of the carbon-carbon links in the hydrocracking reactions.
The operating parameters that determine the degree of cracking severity of the hydrotreating are in particular the hydrogen pressure, the temperature and the duration of the reaction.
The catalysts currently utilized in the hydrotreating are selected from among the hydrogenation catalysts. The hydrogenation catalysts suitable for use in the hydrogenation are those which are not poisoned by sulphur and nitrogen of the hydrocarbon-based charges. Thus, a hydrogenation catalyst as effective as palladium cannot be used in the hydrotreating treatment since it is over-sensitive to the presence of sulphur.
The catalysts most widely used in hydrotreating are cobalt, nickel, molybdenum or tungsten sulphides or their mixtures, generally on an alumina support (DONATI Advances in Catalysis and Related Subjects 8, 39--Academic Press New York--1956).
Due to the increasing use of low quality crudes, containing large quantities of sulphur, nitrogen, metals and asphaltenes, the need has arisen to develop increasingly active hydrotreating catalysts.
In one study, performed with the specific aim of developing novel hydrotreating catalysts, T. A. PECORARO et al. (Journal of Catalysis 67, 430-445, 1981) have examined the activity of a large number of metallic sulphides as hydrosulphuration catalysts. They have observed that the sulphides of the metals belonging to Group VB of the Periodic Table of Elements (vanadium, niobium, tantalum) have a catalytic activity much lower than the sulphides of metals belonging to Group VI (molybdenum, chromium, tungsten). In particular, they found that unsupported vanadium sulphide has no desulphurating activity with respect to dibenzothiophene.
This conclusion has been corroborated by the results obtained by L. A. RANKEL et al. (Fuel 62, 44, January 1983) who compare the reactivity of the classical cobalt-molybedenum catalysts with that of vanadium sulphide. By studying the same reaction as the previous authors, the desulphurization of the dibenzothiophene, they have determined that the activity of the vanadium sulphide is one fifth that of the cobalt-molybdenum sulphides catalyst.
It has now been found that vanadium sesquisulphide (V.sub.2 S.sub.3) prepared in situ in the reaction zone or V.sub.2 S.sub.3 prepared outside the reaction zone and processed in this zone by a sulphurating agent presents a high catalytic activity in the hydrotreating reactions of the hydrocarbon-based charges.
All the reactions leading to the formation of V.sub.2 S.sub.3 are suitable for use in this method. It can involve thermic reactions, V.sub.2 S.sub.3 being obtained through thermic decomposition of a vanadium compound.
Thus, ammonium thiovanadate (NH.sub.4).sub.3 VS.sub.4, can be introduced into the reactor in the form of an aqueous solution. V.sub.2 S.sub.3 is formed through heating this solution at a temperature comprised between 200.degree. and 500.degree. C. V.sub.2 S.sub.3 is obtained in theform of small particles that can be utilized immediately as a catalyst.
It is also possible to impregnate alumina with an ammonium thiovanadate solution and thereafter subject it to the thermic treatment. The V.sub.2 S.sub.3 is formed on the alumina support or on any other porous support.
The V.sub.2 S.sub.3 can also be prepared through hydrogen sulphide reaction with a vanadium compound. Vanadyl sulphate, VOSO.sub.4, is transformed into V.sub.2 S.sub.3, through treating with hydrogen sulphide at a temperature comprised between 250.degree. and 400.degree. C. for at least 30 minutes.
If V.sub.2 S.sub.3 is utilized, formed outside the reaction zone and which has been in contact with air or water vapor, it must be reactivated in the reaction zone by a sulphurating agent. Hydrogen sulphide at a temperature comprised between 250.degree. and 400.degree. C. during at least 30 minutes is generally used.
The V.sub.2 S.sub.3 can be utilized by a hydrotreating catalyst, in its unsupported form or deposited on a porous support such as alumina, silica, magnesia and the clays used either in the pure or mixed form.
The V.sub.2 S.sub.3 can be associated to other catalysts, such as the sulphides belonging to Groups VI B and VIII.
The hydrotreating method according to the invention applies to hydrocarbon-based charges such as gasoline, gasoil, refinery residues as reduced crude or vacuum residues and to heavy crude oil.
The hydrotreating of the gasolines allows one to reduce their sulphur and nitrogen contents as well as to hydrogenize the olefins and diolefins present in these cuts.
With respect to the gasoils or the vacuum distillates, a moderated hydrotreating eliminates more particularly the sulphur, while a high severity hydrotreating leads to hydrocracking with formation of lighter molecules.
In the case of reduced crudes, vacuum residues and heavy crude oils, a low severity hydrotreating eliminates the sulphur and the metals and reduces the asphaltene content. A high severity hydrotreating leads to hydrocracking.
The hydrotreating of the hydrocarbon-based charges according to the present invention is performed at a temperature comprises between 250.degree. and 450.degree. C., a hydrogen/hydrocarbon molar ratio comprises between 100 and 1500 Nl/l, a total pressure comprised between 20 and 200 bars and a liquid hourly space velocity comprised between 0.1 and 10 l/h.
The following examples illustrate the invention, without however limiting the same.